1. Field of the Invention
The present invention relates to a heat sensitive recording sheet having excellent color forming properties, whereby the color images formed are stable against humidity in the room and against stains with oil, fats, etc.
2. Discussion of the Background
Heat sensitive recording sheets are recording sheets having a structure in which a so-called heat sensitive color forming layer capable of forming a color when heated, is formed on a substrate such as paper or the like. A thermal printer internally equipped with a thermal head may be employed for the heating for color forming. Recording sheets having such a structure wherein a heat sensitive color forming layer is provided, are widely used for facsimile printers, etc. for their advantages over other types of recording sheets in that they are inexpensive, they produce less noise, and the recording can thereby be completed in a short period of time. For heat sensitive recording sheets, it is most important that the color forming sensitivity is excellent. In recent years, they have been used also for display labels for POS (point of sale) and have been used in a wide range of fields. For the purpose of improving the basic properties of heat sensitive recording sheets, such as ability to produce sharp color images at high speed, the maintenance of the color images, the minimum initial color development, etc., studies have been made to employ various fluoran compounds as electron donative color formers, and various proposals have been made. However, the results so far obtained are not necessarily satisfactory.
In such heat sensitive recording sheets, it has been known to use a mixture of two or more color formers, for example, in Japanese Unexamined Patent Publications No. 109454/1979, No. 123085/1982 and No. 71192/1983. However, in these cases, heat sensitive recording sheets satisfying both requirements of the maintenance of the color images and the minimum initial color development, have not been obtained due to the nature of the color formers used. Namely, in a heat sensitive recording sheet in which color formers having good fastness against the fading of color images in an environment of heat, humidity, oils, plasticizers or lights, are used in combination as a mixture, the initial color development in the same environment will be unduly high. On the other hand, in a heat sensitive recording sheet in which color formers having minimum initial color development are used in combination as a mixture, the color images obtained, tend to fade away. Further, in a case where a color former having minimum initial color development and a color former having high initial color development are used in combination, the initial color development will be pronounced, such being practically undesirable.